Optical amplifiers are commonly used in optical communication systems. One of the parameters that is important in characterizing an optical amplifier is amplified spontaneous emission (ASE) noise in the presence of an optical signal. The amplified spontaneous emission noise represents a noise signal that is generated within the optical amplifier and is amplified by the amplifier. The ASE noise typically has a wider bandwidth than the optical signal. The measurement of ASE noise is important in determining the noise figure of an optical amplifier as well as ASE buildup in communication systems, where ASE noise can limit performance.
The output of an optical amplifier includes a narrow band optical signal and broader band noise generated within the amplifier. When no signal is present, the amplifier generates and amplifies noise. However, when an optical signal is present, the output noise level is reduced in comparison with the noise level in the absence of an optical signal due to amplifier gain reduction. The gain reduction depends on the amplitude of the optical signal. Thus, in order to accurately characterize amplifier performance, the ASE noise must be measured at an optical signal level and wavelength that corresponds to normal operation.
In accordance with one previously reported technique, referenced here as the delta technique, the ASE noise measurement is performed by measuring ASE noise about the optical carrier, as shown in a spectral display on an optical spectrum analyzer. The actual noise at the optical signal wavelength is then inferred from an interpolation of the measured data. The delta technique is influenced by a combination of finite optical filter selectivity, finite optical signal bandwidth and optical signal sidemodes. These problems become important for large signal levels and for lasers such as distributed feedback (DFB) or distributed Bragg reflector (DBR) lasers, which often have poor sidemode suppression levels.
Another technique for ASE noise measurement is reported by J. Aspell et al in Optical Fiber Communication, Vol. 5, 1992 OSA Technical Digest Series, (Optical Society of America, Washington, D.C. 1992), paper THA4. The ASE noise is measured in the presence of an optical signal by ensuring that the optical signal is polarized and using a polarizer located after the source of ASE noise to reject the signal and pass half of the ASE noise. This technique assumes that the ASE noise levels in each of two orthogonal polarizations are equal. The disclosed technique is relatively complex, and its application to testing ASE noise in the presence of multiple optical signals is questionable.
It is a general object of the present invention to provide improved methods and apparatus for determining the ASE noise of an optical amplifier in the presence of an optical signal.
It is another object of the present invention to provide methods and apparatus for determining ASE noise using a pulsed or gated optical source.
It is a further object of the present invention to provide methods and apparatus for determining ASE noise wherein the deleterious effects of optical signal excess noise and sidemodes are avoided.
It is yet another object of the present invention to provide methods and apparatus for determining ASE noise in the presence of the effects of closely-spaced optical signals.
It is still another object of the present invention to provide methods and apparatus for determining ASE noise using a relatively uncomplicated optical system.